The present invention relates to a semiconductor device and a method for fabricating the semiconductor device, more specifically to a semiconductor device including floating gate electrodes and a method for fabricating the semiconductor device.
As a typical semiconductor memory, DRAM (Dynamic Random Access Memory) is known. DRAM is a semiconductor memory which stores 1-bit information in one memory cell including 1 MISFET and 1 capacitor. DRAM has the memory cells increasingly micronized and the capacities increased. Semiconductor memories which can have larger capacities are expected.
As a semiconductor memory which can increase capacities, flash memories are noted. A flash memory includes memory cells each constituted by only one MISFET, which makes the flash memory suitable for the capacity increase.
In the flash memory, information is stored by injecting carriers into the floating gate electrodes of floating gate-type FETs. For retaining carriers injected in the floating gate electrodes, an insulation film between the floating gate electrodes and the channel region must have a thickness of above about 8 nm.
When carriers are injected into a floating gate electrode, a high voltage is applied between the channel and the floating gate electrode. When a high voltage is applied between the channel region and the floating gate electrode, FN (Fowler-Nordheim) tunnel phenomenon causes carriers to be injected into the floating gate electrode.
However, to inject carriers into a floating gate electrode by FN tunnel phenomenon, a 10 to 20 V voltage is necessary. The conventional flash memory requires application of high voltages, with a result of high electric power consumption.
To enable writing at low voltages it is considered effective to thin a tunnel insulation film between the channel region and the floating gate electrode. Merely thinning the tunnel insulation film, however, allows electrons injected into the floating gate electrode to easily move into the channel region, which makes it difficult to retain information in the floating gate electrode for a long period of time.
An object of the present invention is to provide a semiconductor device which can retain information for a long period of time even in a case that the tunnel insulation film is thin, and a method for fabricating the semiconductor device.
The above-described object is achieved by a semiconductor device comprising a first insulation film formed on a semiconductor substrate, a floating gate electrode formed on the first insulation film, a second insulation film formed on the floating gate electrode, and a control gate electrode formed on the second insulation film, a depletion layer being formed in the floating gate electrode near the first insulation film in a state that no voltage is applied between the floating gate electrode and the semiconductor substrate. Even in a case that the tunnel insulation film is thin, recombination of the carriers can be prohibited, and the semiconductor device can retain information in the floating gate electrode for a long period of time.
The above-described object is achieved by a method for fabricating a semiconductor device comprising the steps of: forming a first insulation film on a semiconductor substrate; forming a floating gate electrode on the first insulation film; forming a second insulation film on the floating gate electrode; and forming a control gate electrode on the second insulation film, in the step of forming the floating gate electrode, the floating gate electrode being formed so that a depletion layer is formed in the floating gate electrode near the first insulation film in a state that no voltage is applied between the control gate electrode and the semiconductor substrate. Even in a case that the tunnel insulation film is thin, recombination of the carriers can be prohibited, and the semiconductor device which can retain information in the floating gate electrode for a long period of time can be fabricated.
According to the present invention, an impurity concentration of the floating gate electrode is lower near the tunnel insulation film, whereby a thick depletion layer can be formed in the floating gate electrode near the tunnel insulation film. When a thick depletion layer is formed, a conduction band of the floating gate electrode from a valence band of the semiconductor substrate are spaced from each other by the tunnel insulation film and the thick depletion layer, whereby the carriers accumulated in the floating gate electrode cannot easily tunnel toward the semiconductor substrate. Thus, according to the present invention, recombination of the carriers can be prohibited even in a case that the tunnel insulation film is thin, whereby the semiconductor device according to the present invention can retain information in the floating gate electrode for a long period of time.